array.array.tostring is deprecated in Python 3 in favour of tobytes

Following instances should use tostring on Python 2 and tobytes on Python 3.

```
test/test_encoding.py
114:        encoded_bitstring = array.array('B', raw_data_in).tostring()
134:            'B', [0b00000101, 0b00111001, 0b01110111]).tostring()
```